Recent experimental results in this laboratory suggest that pluripotential hemopoietic stem cells, when transfused into normal isogeneic mice, proliferate temporarily in the spleen, but do not reach the marrow in substantial numbers. In contrast, irradiated mice restored to hematological normality by marrow transfusion subsequently allow proliferation and differentiation of transfused isogeneic but not allogeneic stem cells in their marrow in significant numbers. (Both situations contrast with the ready proliferation of allogeneic stem cells in irradiated mice not restored to hematologic normality.) The mechanisms responsible for the observed differences in the behavior of transfused marrow stem cells in normal and in previously irradiated mice will be investigated. The studies should elucidate the nature of the selective distribution ("homing") of transfused stem cells and the local control of their proliferation. The results may be of clinical significance if present attempts to completely prevent graft vs. host reaction should be successful.